1. Field of the Invention
The present invention relates to a method of data transmission management for use in, for example, a bus system interconnecting a plurality of instruments.
2. Description of the Related Art
The digital interface, known as IEEE1394, has been proposed as a network which is suited for real-time transmission of audio and video data (referred to as AV data hereinafter) of the MPEG2 transport packet form (referred to as TS packet hereinafter) or the digital video format (referred as DV hereinafter). The IEEE1394 is a serial high-speed bus system which transmits data in isochronous format at synchronous timing, hence enabling real-time transmission of the data. Also, asynchronous data transmission at asynchronous timing can be made in parallel with the isochronous data transmission.
The IEEE1394 permits a maximum of 63 instruments to be connected over a single local bus. The cable of each instrument can be connected or disconnected while the instrument is being energized. Upon an instrument being applied or removed, its connection can automatically be identified to reconstruct the network.
The IEEE1394 can be installed as an external interface in a variety of digital audio and video instruments. A combination of the IEEE1394 and its AV data transmission mode or AV protocol (specified in IEC 61883: Specifications of digital interface for consumer electronic equipment) permits, for example, two DV instruments to be communicated each other for data transmission and digital dubbing. Also, any other asynchronous transmission mode than the AV protocol may be used such as AV/C digital interface command set or asynchronous connection for delivering control commands, including Play and Stop, to AV instruments.
In the IEEE1394, multi-channel AV signals can be transmitted over a single connector. Also, both the input and output of a signal can be carried out through a single connector. The AV protocol assigns a plug as an imaginary connector for receiving or transmitting a single channel. Hence, two types of plugs are needed for input and output respectively; the input plug receives data on a single channel and the output plug transmits data on a single channel. This requires an instrument to have N input plugs for receiving N channels of the isochronous packet and have N output plugs for transmitting N channels of the isochronous packet.
A register (PCR, plug control register) is provided for assigning the channel to be handled or determining the on/off action. The PCR is classified into two types, iPCR for input and oPCR for output. The PCR is located in an address area of the IEEE1394 and can thus be read out directly using an asynchronous packet or modified in the value. Hence, no dedicated control commands are needed for controlling input and output of a signal and the PCR is also embodied in a hardware.
The PCR is thus utilized in a connection management procedure (CMP) of the AV protocol for signal connection between instruments. The CMP is classified into three different signal connection methods: 1) broadcast out connection, 2) broadcast in connection, and 3) point-to-point (p-to-p) connection. Their concepts are illustrated in FIG. 4. FIG. 4 shows an exemplary bus system of plugs where a camcorder 0, a VCR 1, a VCR 2, an STB (set top box, a satellite broadcast receiver) 3, a monitor 4, and a controller 5 are interconnected by the IEEE1394 bus as shown in FIG. 10.
The broadcast out connection is established between an output plug and a channel. Referring to FIG. 4A, the camcorder 0 is linked by the broadcast out connection to the 63rd channel. The 63rd channel and the monitor 4 are linked by the broadcast in connection. Accordingly, data from the camcorder 0 is received by the monitor 4 over the channel as a medium.
The broadcast out connection may be broken by any other instrument. For instance, when its PLAY button switch of the VCR 2 is pressed by a user, the VCR 2 may break the broadcast out connection from the camcorder 0 and run an application of establishing a broadcast out connection to the 63rd channel. In this case, the monitor 4 is linked to the 63rd channel by the broadcast in connection and thus its displaying-image is automatically shifted to an image from the VCR 2.
The VCR 2 detects that the 63rd channel to which its image is released is linked to another instrument (the camcorder 0) and reads out an oPCR at the instrument. When also acknowledging that the 63rd channel engages simply the broadcast out connection, the VCR 2 breaks the broadcast out connection by rewriting the oPCR.
As a result, the camcorder 0 stops releasing the isochronous packet to the 63rd channel and thus allows the VCR 2 to establish the broadcast out connection to the 63rd channel.
When a protected signal connection between two dedicated instruments is desired, it employs the p-to-p connection. As shown in FIG. 4, the p-to-p connection between the STB 3 and the VCR 1 is established over the 61st channel. The p-to-p connection is a one-to-one connection which can hence be broken only by the instrument which established the connection. The p-to-p connection may be established by an output instrument, an input instrument, or a third instrument. For example, the p-to-p connection between a playback VCR and a recording VCR may be established by the controller 5. Also, two or more of the p-to-p connections may be overlaid with a single output plug. Moreover, two or more of the p-to-p connections may be overlaid with a single input plug or two or more input plugs.
However, the above mentioned connection management method of the CMP has following drawbacks.
First, the p-to-p connection may create useless isochronous transmission. Characteristic examples of the useless isochronous transmission are:
1-1) that the p-to-p connection which is established by a third instrument may continuously be maintained while no actual data is transmitted or empty packets are delivered. Also, the p-to-p connection which is established by a third instrument or a transmitter may be maintained while the transmitted data is not used by the receiver. The isochronous transmission assigns a bandwidth before starting the transmission and, when the transmission is finished, restores the bandwidth. A resultant case may be that the bandwidth is allocated to another instrument hence disabling the isochronous transmission from the transmitter. The p-to-p connection may lead to useless data transmission, thus disturb the effective data transmission;
1-2) that the data received over the p-to-p connection established by a transmitter or a third instrument may not always favorably be used by a receiver; and
1-3) that the data transmitted over the p-to-p connection established by a receiver or a third instrument may not always be guaranteed by a transmitter. This will result in useless data transmission.
Secondly, the broadcast out connection does not identify a receiver and may hence permit no instruments to be responsible to data receipt. Accordingly, such useless data transmission will decline the available overall transmission capability in a limited resource for data transmission.
Thirdly, any instrument which intends to establish the broadcast in connection is not contemplated for finding an instrument which is ready for establishing the broadcast out connection. For example, as the receiver is not informed of any instrument which is ready to transmit data before actually receiving the data, it has to examine channels over which the data of interest is transmitted.